Voltage regulators are widely used circuitry in any integrated circuit (or chip). When an external power is input to an integrated circuit, its characteristics are typically unstable, for example having an unpredictable voltage level or a high level of noise because of interference from the surrounding environment. An on-chip voltage regulator is used to regulate the external power by removing fluctuation and noise, thereby providing a regulated stable voltage to the other on-chip circuits that use the voltage. That is, the voltage regulator operates to isolate the on-chip circuits from the fluctuation and noise in the external power.
FIG. 1 illustrates the configuration of a voltage regulator 100 in accordance with the prior art. The voltage regulator 100 includes an error amplifier 101 and a pass transistor 102 which serves as a passing element to pass current from an unregulated voltage VDD_RAW1 to the voltage regulator output 104. The voltage regulator 100 may further include a feedback network comprised of Rload1 and Rload2 103A, 103B for current limiting. The voltage across the Rload2 103B is the regulated output voltage VDD_REG 104 which is supplied to the application circuitry as well as to the amplifier 101 through a feedback path 105. Some implementations may have an Rload1=0. Alternatively, some implementations may have either Rload1=0 and omit Rload2.
During operation, the amplifier 101 compares the output of a reference voltage Vref to the output voltage VDD_REG supplied through the feedback path 105. The output of the error amplifier 101 is coupled to the gate node 106 of the NMOS pass transistor 102, the gate node being used as a control terminal of the pass transistor 102. The amplifier 101, the pass transistor 102 and the feedback load network including Rload1 and Rload2 103A and 103B form a feedback control loop acting to force the control terminal 106 of the pass transistor 102 to a voltage that can maintain a regulated voltage VDD_REG at the output terminal 104 of the voltage regulator 100.
In this configuration, the regulated voltage VDD_REG is required to be lower than the unregulated voltage VDD_RAW1 to provide the regulation and isolation between the VDD_REG and VDD_RAW1. Hence the voltage range that can be supplied by the voltage regulator is constrained by the VDD_RAW1 and the drain source voltage requirement of the pass transistor 102. For example, given the VDD_RAW1 equal to 1.0 V and a drain source voltage requirement of 0.15V, the output regulated voltage is limited to about 0.85V.